Hand-held syringes have long been used for injecting chickens with antibiotics and the like. The risks and dangers of accidental injection of the operators have led to development of mechanized injection equipment such as that described in co-pending patent application Ser. No. 06/779,221 filed Sept. 24, 1985 by Gordon J. Lancaster, Jr. et al. When a conventional syringe is used in mechanized injection equipment, it has been found that air can become trapped in the compression chamber of the syringe, with the result that the power stroke of the syringe piston merely compresses the air instead of forcing the treating liquid out through the injecting needle, and the return stroke fails to develop the reduced pressure needed to recharge the syringe. A like problem can be encountered in the injector pump of a diesel engine.